The goal of this R21 proposal is to obtain a high-resolution crystal structure of the angiotensin II type 1 receptor (AT1R). The AT1R is a member of the Class A G protein coupled receptor (GPCR) superfamily that plays important roles in the regulation of cardiovascular function. Drugs acting on the AT1R are currently used in the treatment of hypertension and heart failure. There are three distinct classes of drugs for the AT1aR: (1) classical antagonists, angiotensin receptor blockers (ARBs) that stabilize the receptor in an inactive conformation, (2) the endogenous agonist angiotensin II that stabilizes an active conformation of the receptor capable of signaling through both the G proteins and the ? -arrestins, and (3) a highly specific ? -arrestin biased agonist that stabilizes a conformation of the receptor that is capable of signaling exclusively through ? -arrestins without any detectable activation of G proteins. Such ??arrestin biased agonists have unique pharmacological and therapeutic properties, distinct from classical agonists or antagonists, e.g. they lower blood pressure (like antagonists) but increase cardiac performance (like agonists). Little is known about the structural basis by which these different types of ligands regulate receptor function. We propose to begin a detailed investigation of the structural biology of the AT1aR through an R21 mechanism. Our goal for this proposal is to demonstrate that we can obtain diffraction quality crystals and a high-resolution structure of the AT1R bound to a high-affinity antagonist. The proposed work is high-risk and high-impact. If successful, the outcome of this R21 proposal will enable us to obtain funding for a more thorough structural characterization of the AT1aR in different conformational states. The long-term objective of this proposal is to determine the high-resolution crystal structures of the AT1aR in three different conformations; the inactive conformation stabilized by an ARB; the classical active conformation stabilized by angiotensin II; and an active conformation capable of signaling through only 2- arrestins stabilized by a ? -arrestin biased agonist. These structures will facilitate the development of safer and more effective therapeutics for heart failure and hypertension. Specific Aims include: 1) Generate an AT1aR-T4lysozyme fusion protein and adjust the linkers between these two proteins to optimize AT1aR functional expression and stability. 2) Establish conditions for expression and purification of AT1aR for crystallography trials. 3) Crystallize and determine the X-ray crystal structure of the AT1aR.